Python enum auto

Summary: in this tutorial, you’ll learn about the enum auto() function to generate unique values for enumeration members.

Introduction to the enum auto() function

The following example defines an enumeration with three members whose values are 1, 2, and 3:

from enum import Enum class State(Enum): PENDING = 1 FULFILLED = 2 REJECTED = 3
Code language: Python (python)

In this example, we manually assign integer values to the members of the enumeration.

To make it more convenient, Python 3.6 introduced the auto() helper class in the enum module, which automatically generates unique values for the enumeration members. For example:

from enum import Enum, auto class State(Enum): PENDING = auto() FULFILLED = auto() REJECTED = auto() def __str__(self): return f'{self.name(self.value)}'
Code language: Python (python)

How it works.

  • First, import the Enum and auto classes from the enum module.
  • Second, call the auto() to generate a unique value for each member of the State enumeration.

By default, the auto() class generates a sequence of integer numbers starting from 1.

The following shows the values of the State enumeration’s members:

for state in State: print(state.name, state.value)
Code language: Python (python)

Output:

PENDING 1 FULFILLED 2 REJECTED 3
Code language: Python (python)

How enum() auto works

Technically, the auto() calls the _generate_next_value_() method to generate values for the members. Here’s the syntax of the _generate_next_value_() method:

_generate_next_value_(name, start, count, last_values)
Code language: Python (python)

The _generate_next_value_() has the following parameters:

  • name is the member’s name
  • start is the starting value of the enum members.
  • count is the number of enum members, including aliases, that have been created.
  • last_values is a list of all preceding values used for the enum members.

By default, the _generate_next_value_() generates the next number in a sequence of integers starting from one. However, Python may change this logic in the future.

It’s possible to override the _generate_next_value_() method to add a custom logic that generates unique values. If so, you need to place the _generate_next_value_() method before defining all the members.

The following shows how to override the _generate_next_value_() method to generate values for members by using their names:

from enum import Enum, auto class State(Enum): def _generate_next_value_(name, start, count, last_values): return name.lower() PENDING = auto() FULFILLED = auto() REJECTED = auto() for state in State: print(state.name, state.value)
Code language: Python (python)

Output:

PENDING pending FULFILLED fulfilled REJECTED rejected
Code language: Python (python)

Summary

  • Use enum auto() class to generate unique values for enumeration members.
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